Process for making pressed fiberboard utilizing through circulation preheating



Sept. 23, 1969 c, cuL

PROCESS FOR MAKING PRESSED FIBERBOARD UTILIZING THROUGH CIRCULATION PREHEATING Filed Aug 16. 1966 R M "W m w w M R a 4 s $55K ma: 553% 58% cu 2E: w 3688mm u EIEE v A W 3 om vm O 0 W O 0 0 q x v x a 2 mm 25% 2Q 25; @252 $252 sa E5; :1 928 HIIL..\ a O 0 Kw? m; 255 z s H O c 1 X n v 0. 2m :0 a a 2w 5; 5% E E5 232 52w 5;

United States Patent ,0

US. Cl. 162-206 4 Claims ABSTRACT OF THE DISCLOSURE A process for making pressed fiberboard that accomplishes a preheating step by circulating warm air through the boards. The circulation of the warm air through the boards is carried out by maintaining a pressure differential on either side of the boards, with the pressure on one side thereof being subatmospheric to assure that the warm air travels through the boards before being exhausted.

This invention relates to improvements in the art of making pressed boards, such as fiberboard, and particularly to the utilization of through circulation preheating in such a process.

In the process of making fiberboard, it is presently known to form a slurry into a wet fiber mat, remove moisture from the mat, for example, by wet pressing, then cut the mat into boards and finally press the boards with sufficient heat and pressure to densify the boards. The final pressing usually takes place in a platen type press with a considerable portion of the press time required to raise the boards to the desired temperature. This invention improves the process by preheating prior to pressing by passing hot gases directly through the board. Such preheating lessens the time in the presses to thus provide a greater capacity per unit of equipment by virtually reducing the moisture content in the boards when they reach the press.

Not only does this invention envision preheating of the boards prior to the pressing step in the process, but this preheating must be by through circulation. That is, a drying gas is circulated completely through a board prior to pressing, and the gas has a temperature suflicient to raise the temperature of the core of the board to between 245 F.430 F. Through circulation preheating has significant benefits over ordinary convection heating in an oven in that it is quicker and produces a more uniformly preheated and dried product prior to the pressing operation. Also, embrittlement of the fibers of which the board is composed occurs through prolonged exposure to excessive heat which would occur if a drying oven were used for preheating to the degree necessary prior to pressmg.

A problem in the manufacture of fiberboards in the past has been the possibility of board explosion within the press due to moisture water in the board, and as the board is raised above the steam temperature, the board actually ruptures and explodes. By preheating the board, such explosions can be eliminated or at least the potential of explosions can be greatly reduced.

Additionally, in the manufacture of pressed fiberboard, it is desired that the board product have dimensional stability, low water absorption and high transverse strength. It has been found that by utilizing through circulation preheating prior to pressing in the process all of these desirable product improvements can be achieved.

The foregoing and other features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.

In the drawing:

The single figure is a schematic illustration of the process of this invention.

Referring to the drawing, a wet slurry of the type usually used and suitable for making pressed fiberboard is contained in a tank 10 and is formed on a cylinder forming machine 12 into a wet fiberboard mat 14. This mat is pressed in a wet press 16 to remove a portion of the moisture therefrom and is then sawed into relatively large pieces by a wet saw 18. These large wet boards 20 are loaded into multiple decks 22 of a board dryer 24. The boards from the board dryer have most of the moisture removed therefrom and are then sawed into smaller pieces by a dry saw 26 and these smaller boards 28 may be loaded on pallets or the like, and in the conventional process taken to a platen press 30. In the conventional prior art, the boards are heated and pressed in the platen press 30 but the resultant product has some deficiencies with regard to its modulus of rupture, that is, transverse strength, water absorption, dimensional stability, and in addition, the platen press takes a relatively long period of time to bring the temperature of the boards up to that required. Also, undue moisture within the board may, when heated to steam, cause an explosion within the board rupturing and running the board.

In order to eliminate the aforementioned problems, a through circulation preheater 32 is provided for preheating the boards 28 by hot drying gas circulated through the fiberboards. A board may be enclosed in a chamber 34 in which gas burners 36 discharge their products of combustion into the top half of the chamber and a vacuum drawn by vacuum pump 38 draws the hot drying gas through the board in the chamber as shown by the arrows in the figure. One board at a time may be in the preheater for a predetermined preheat time and the vacuum pump 38 will draw a predetermined vacuum across the board and the temperature of the gases will be a certain amount in order to heat the core of the board to between 245 F.-430 F. Thereafter, the preheated board shown at 40 remains for a certain dwell time to enable it to be loaded into the multiple platen press 30 for its final densifying and pressing operation prior to further processing, such as additional trimming or cutting to size, decoration, etc.

The various parameters of the air temperature, air flow, vacuum drop across the board, board temperature, preheat time and dwell time may be summarized as fol lows: The drying gas temperature will be a maximum of 450 F. Above 450 F. the danger of combustion becomes more prevalent and below 260 F. the heating value of the gas becomes marginal. The thickness of the board as it is fed to the preheater prior to the densifying operation is usually between /2 and 1 /2 inches. The air flow through the board is between 20 cubic feet per minute per square foot of board and cubic feet per minute per square foot of board with a vacuum drop of between 4 and 8 inches in mercury. The core temperature of the board is, as mentioned above, between 245 F.-430 F., minimum and maximum and the preheat time is between 60 seconds and 180 seconds. The dwell time after the preheating and before the board is loaded into the platen press is a minimum of 3 minutes to a maximum of 1 hour.

As examples, using similar rough stock pressed at the same time and for the same press cycle of approximately 9 minutes, the comparison of water absorption, as a percent of the weight, and the modules of rupture for 40 pounds per cubic foot of board with through circulation preheat and with no preheat, may be summarized as follows:

Water Absorption, Percent Modulus of Rup- Process Weigh ture 1 No preheat 14. 1 1 1,800 2 Through circulation preheat 12. 1 2, 200 Percent improvement 14. 7 22. 2

1 Per square inch.

Additional examples operated on a different type of pulp (cold caustic), first with an approximately 6 minute press cycle are as follows:

Water Absorp- Modulus of Ruption, Percent ture (For 40 lbs./ Process Weight cu. ft. board) 1 No preheat 11.3 1 2, 000 2 Through circulation preheat 6. 6 1 2, 280

1 Per square inch. Secondly, with an approximately 4 minute press cycle:

Water Absorp- Modulus of Ruption, Percent ture (For 40 lbs./

Process Weight cu. it. board) 1 N o preheat 17. 7 1, 860 2 Through circulation preheat 9. 2 2, 200

4 various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a method for making pressed fiberboard including forming a slurry into a wet fiber mat, removing moisture from the mat, cutting the mat into boards and then pressing the boards under heat and pressure to density the boards, the improvements comprising; circulating hot drying gas at a temperature of 260 to 450 F. through the board by maintaining a pressure diiTerenti-al across the thickness of the board for a sufiicient time to raise the temperature of the core of the board to above 245 F.

2. A method as in claim 1 wherein the drying gas is circulated for sufiicient time to raise the temperature of the core of the board to between 245 to 430 F.

3. A method as in claim 1 wherein the time of preheating by circulating the drying gas through the board is between and seconds.

4. A method as in claim 1 wherein the pressure differential maintained across the thickness of the board is between 4 to 8 inches of mercury.

References Cited UNITED STATES PATENTS 2,140,189 12/1938 Mason 162206 X 2,167,440 7/1939 Mason 162-206 3,002,878 10/1961 Linzell 162-225 X S. LEON BASHORE, Primary Examiner T. G. FERRIS, Assistant Examiner U.S. Cl. X.R. 

